Printed circuit boards for producing electronic circuits carry and connect electronic components, compactness and high interconnection density being sought. It therefore forms part of the prior art to directly solder onto the surface connection layer of printed circuit boards integrated circuits in the form of surface mounted devices (abbreviated SMD) or leadless chip carriers (abbreviated LCC). The printed circuit boards generally have a multilayer construction, i.e. they have several conductor layers.
The reliability of SMDs soldered onto a printed circuit board is greatly reduced if the thermal expansion coefficient of the components differs significantly from that of the printed circuit board. Different thermal expansions of the board and components leads to high shear stresses at the soldered joints, which can then lead to cracks. The problem does not occur to the same extent when using components having metal leads, because the latter serve as flexible intermediate members between the board and the component and in this way the stresses can be compensated.
As a function of the materials used, conventional printed circuit boards have a thermal expansion coefficient of 15 to 20 ppm/.degree. C., but the coefficient approximately 6 ppm/.degree. C. for components in ceramic packages without leads, i.e. leadless ceramic chip carriers (LCCC). The use of LCCC's consequently requires printed circuit boards whose thermal expansion is reduced by suitable measures, so that the stresses are limited to an acceptable amount.
Standard measures for reducing the thermal expansion of printed circuit boards are the fitting of cores or intermediate layers of thermally stable materials. For example metal sheets or foils formed from a combination of copper and an alloy sold under the trademark INVAR.TM. (an alloy of nickel and iron with a very low expansion coefficient), of molybdenum (with a thermal expansion coefficient of approximately 5 ppm/.degree. C.) or cores with a carbon fiber reinforcement are known. Further information on this problem are provided by EP-A-393312 which is assigned to the assignee of this application.
As a result of these measures the composite of thermally stable reinforcement and conductor layers, i.e. the printed circuit board, acquires an adapted, low expansion factor and in this way the thermal stresses at the soldered joints are reduced. However, it leads to a new problem. Because of temperature changes, shear stresses act at the interface between the cores or intermediate layers of the thermally more stable materials and the remaining layers of the printed circuit board. This occurs because the cores or intermediate layers, due to their high mechanical strength, force the board to remain dimensionally stable, whereas the remaining layers of the board, having the tendency to expand or contract more with the temperature change, are subject to a restriction of their freedom of movement because of their adhesive attachment to the cores or intermediate layers. If untreated intermediate layers are laminated together with conductor layers to form a printed circuit board, due to the normal temperature cycles, delamination must be expected, i.e. the union between the conductor layer and the intermediate layer is broken. This separation can in particular be observed if glass fibre-reinforced polyimide resin is used as the laminate base material. However, poor results are also obtained with base materials from cyanate ester resins.
Solutions for overcoming this problem are also known. The surfaces of the metal foils (copper/INVAR.TM., copper/molybdenum) are pretreated by black oxidation, sand blasting or the application of a treatment coating. The normally used black oxidation is admittedly sufficient to prevent the delamination of glass fibre-reinforced epoxy materials, but is not sufficient in the case of polyimide or cyanate ester resin base materials. The necessary adhesion coefficients are also not provided by sand blasting. There also is a process in which an additional coating, normally of copper, is so electrodeposited on the stabilizing intermediate layer, that a rough surface is obtained. However, this process is complicated and expensive. It is always used over the entire surface, so that no desired or even necessary recesses are possible. The product also suffers from the disadvantage of being sensitive to pressure and scratching and cannot subsequently undergo photochemical processing.